1. Field of the Invention
The present invention relates to a rechargeable battery module and a battery charging method.
2. Description of the Related Art
A lithium battery is small-sized, light weight and rechargeable. Lithium batteries are often packaged in series as a rechargeable battery module used in electronic devices.
However, an over charged lithium battery may produce gas, and there is a risk of explosion. Thus, the voltage of every battery cell of a rechargeable battery module has to be monitored. A charging procedure performed on a series of the battery cells has to be stopped once the voltage of any battery cell reaches an overcharge voltage threshold. The charge capacity of the entire rechargeable battery module, therefore, is seriously reduced.
A conventional battery balancing procedure, generally, is performed when the rechargeable battery module is charged in a constant current mode (CC mode). Considering the constant charging current, a voltage measured between the two ends of a battery cell includes a voltage drop I*R generated by the internal resistance of the battery cell. After a period of time of use, aging of each the battery cells may be different, which result in unequal internal resistances. Different battery cells may have different voltage drop I*R during battery balancing procedure. Thus, it is difficult to make a real balance between the different battery cells by conventional battery balancing techniques.